In recent years, with still more expanding expectations for development of an optical communication technique to provide high bit rates, large capacity transmission path as an infrastructure of the information-oriented society, there have been promoted global and vigorous researches and developments of high rate, large capacity optical communication systems.
On land, an optical wavelength division multiplexing transmission system of a 10 Gb/s transmission rate using a transmission path of a 1.3 μm band single mode fiber (SMF) and a 1.55 μm band dispersion shift fiber (DFS) has come into practice.
Under the ocean, on the other hand, an optical wavelength division multiplexing transmission system of a 10 Gb/s transmission rate using a transmission path of a non-zero dispersion shift fiber having a zero-dispersion wavelength at a 1.58 μm band has come into practice.
Generally, a transmission waveform deterioration occurs in the optical fiber due to interaction (called SPM-GVD effect) between a self-phase modulation (SPM) and a group-velocity dispersion (GVD). Therefore, a possibly smaller value should be set as a value of the (group-velocity) dispersion to be caused by a difference in transmission time in optical fiber between optical signals of different wavelength.
However, as the wavelengths of the optical signals approach a zero-dispersion wavelength, there is an increased tendency for a four-wave mixing (FWM) to cause a crosstalk, with an increased deterioration of transmission characteristic. Therefore, the optical wavelength division multiplexing transmission requires a wavelength layout in consideration of a zero-dispersion wavelength of optical fiber.
An optical wavelength division multiplexing transmission system is disclosed in Japanese Patent Application Laid-Open Publication No. 8-97771. This conventional system uses a transmission path of a 1.55 μm band dispersion shift fiber (DSF), in which the effect of a conventional four-optical-wave mixing is suppressed.
FIG. 17 is a graph describing the principle of the conventional system, in which signals of respective wavelengths are subject to a wavelength conversion for an interchange between wavelengths to make the optical level of crosstalk lower than specified.
At present, if two different networks (transmission paths), such as for land use and submarine use, are to be connected at a connection point therebetween, then the two different networks are electrically terminated. However, it is necessary for aiming at a practical low-cost system to implement a connection-less simplified structure for structural integration of two networks.
In the conventional system, however, optical fibers in use have different zero-dispersion wavelengths, and a direct use for transmission accompanies a crosstalk due to the SPM-GVD effect or FWM, as a significant problem.